Half Carrier Spurs on RFPA design for MRF series

[danesh]

Hi Guys,

Im working RFPA development for UHF 2 design using MRF1570 freescale part. When conducting VSWR 3:1 load pull to check the stability i found for some angle there is Half Carrier Spurs. Spurs with delta frequency of 1/2MHz from un-modulated carrier. But the spurs reduced once bias level for the PA was reduced from 200mA to 15mA.
1) Why does the PA performance in term of stability becomes better when operating at near cuttoff region ?
2) Any reason for half carrier spurs to happen?

 

[vfone]

Actually all the performances specified by Freescale for this transistor are mentioned at Idq greater than 600mA (800mA optimal).

 

[tony_lth]

1. When PA is at cutoff region, surely all the signal will decrease greatly. someone even say, no current, no spurs.
2. At some angle, the load is different, so the VSWR maybe worse, maybe amplifier self oscillation. So you can try to paste-and-try some copper or change the value of L or C to diminish the self oscillation. You can also try solder 1pF between the amplifier output root and GND.
You can also try to use signal generator replacing LO or IF signal, make LO decrease 1M, and see the spur change or not, so you can decide it's spur or self oscillation.

 

[zltogo]

This is sub harmonic oscillation. When you meet this oscillation, If you reduce the Vdd, it will disapear.

 

[RCinFLA]

PA's have some self biasing from input RF. The bias circuitry bypassing creates a possible source of oscillation, or 'squigging' of the average developed bias.

 

[danesh]

PA's have some self biasing from input RF. The bias circuitry bypassing creates a possible source of oscillation, or 'squigging' of the average developed bias.

1) What could i do in term of bypass circuit in gate ?
2) Any article on this?

 

[vfone]

If you follow the layout and the bypass capacitors values (and placement) recommended by Freescale in the datasheet, you don't have to do more investigation.